This invention relates generally to centrifugal machines and in particular to a variable width diffuser for use in a centrifugal machine.
Centrifugal compressors used in refrigeration systems are generally required to operate over a relatively large flow range. The efficiency and stability of the compressor, to a large extent, is dependent upon the diffuser's ability to convert kinetic energy contained in the working fluid leaving the impeller into static pressure. As the load on the machine changes, the volumetric rate of flow through the diffuser correspondingly changes. With a fixed diffuser geometry, the flow through the diffuser passage becomes unstable as the flow rate decreases below a certain level. Further reduction in the flow rate leads to a surge condition whereupon the working fluids undergo periodic flow reversals in the diffuser passage. This, of course, creates a good deal of unwanted noise and destroys the efficiency of the machine. If the rate of flow through the machine increases, the diffuser will be incapable of handling the flow through the fixed passage and a choke condition is soon reached which again adversely affects machine performance and efficiency.
Many schemes have been devised to maintain high machine efficiencies over a wide operation range. In U.S. Pat. No. 4,070,123, the entire impeller wheel configuration is varied in response to load changes in an effort to match the machine performance with the changing load demands. Adjustable diffuser flow restrictors are also described in U.S. Pat. No. 3,362,625 which serve to regulate the flow within the diffuser in an effort to improve stability at low volumetric flow rates. Variable diffuser vanes as disclosed in U.S. Pat. No. 3,957,392 are used for the same purpose. In U.S. Pat. No. 3,251,539, a centrifugal refrigerant compressor is described having a movable diffuser wall that is used to change the width of the diffuser passage. The width of the diffuser passages is changed in response to changes made in the position of compressor inlet guide vanes. Again, by matching the geometry of the diffuser to the inlet flow, surging at low flow rates is avoided. A similar device is also shown in U.S. Pat. No. 4,219,305.
One effective technique for maintaining high operating efficiency over a wide flow range in a centrifugal machine is through use of the variable width diffuser in conjunction with fixed diffuser guide vanes. This type of arrangement is shown in U.S. Pat. Nos. 2,996,996 and 4,378,194 as well as in British Pat. No. 305,214. In these arrangements, the diffuser vanes are securely affixed, as by welding, to one of the opposed diffuser walls. The vanes are adapted to pass through openings formed in the other wall thus permitting the geometry of diffuser to be changed in response to changing load conditions. Fixedly mounting the diffuser blades to one of the diffuser walls presents a number of problems particularly in regard to the manufacture, maintenance and operation of the machine. Little space is afforded for securing the vanes in assembly. Any misalignment of the vanes will cause the vane to bind or rub against the opposite wall as it is being repositioned. Similarly, if one or more vanes in the series has to be replaced in assembly, the entire machine generally must be torn down to effect the replacement. This requires a good deal of down time and is costly. The receiving opening, through which the vanes pass are sometimes made overly large to avoid alignment problems. This can produce unwanted loss of working fluids and pressure variations in the diffuser region which again adversely affects performance. Lastly, the machine is generally exposed to thermal growth in the course of normal operations. The magnitude of growth may exceed manufacturing tolerance resulting in binding and/or rubbing problems as the width of the diffuser passage is being changed to meet changing load conditions.